Here's a Dr. House mystery:

I've had a dynohub setup for about three years now. Sanyo H-27 hooked up to a B&M HL Lumotec Lyt N Plus which runs back to a B&M Toplight Line Plus Taillight. Both lights were mounted on Jandd Extreme front and rear racks, respectively–great touring and commuting setup.

After my most recent tour (Pittsburgh to D.C.––highly recommended!), I upgraded the front light to a B&M Luxos U LED headlight. A few weeks later I noticed that the rear light indicator was not firing, and the rear light was only going on sporadically.

I inspected the wiring and found that it looks like it had melted through the insulation where I had the wire curving around the head tube.

Thinking the Luxos was outputting too much power, I sent it back to Germany for replacement through warranty, and reinstalled the Lyt N Plus.

Last week I noticed the same pattern of melting starting to occur, about six inches behind the light where the wire was twisted around the head tube twice before following the brake cable to the rear of the bike. For now I've separated the positive and negative strands and wrapped each individually in electrical tape as a stop-gap measure.

While I haven't yet switched back to the Luxos, this occurrence likely eliminates the front light as being the variable causing the melting issue.

In trying to think of what else could have caused both taillight wires to melt through their insulation, the following variables were changed after my tour:

1. The Sanyo was mounted backwards for about two weeks. DON'T DO THIS! The hub's internals start loosening themselves, and I was getting a lot of play in the hub. Local shops couldn't figure it out (very few shops know much about dynohubs), until I did some research on my own. I flipped the wheel back so that the plug was on the drive side, and luckily the hub started tightening back up again. After a half mile of riding there was no more play in the hub. This should not have affected the hub's electrical output, however.

2. I want to switch to a different front rack, so for now the front light is mounted to the brake bolt, and not to the front rack as it was previously. The taillight wire wraps around the head tube twice before following the same path along the top tube brake cable as before back to the taillight, as opposed to wrapping around the head tube once, then passing under the front rack to the head light. It seems highly unlikely that an aluminum frame ('85 Cannondale ST500) could create magnetic induction, but both times the melting has occurred in this area.

So, Sherlocks of Bike Forums, who has any ideas of what's causing the melting and how to stop it? Has anybody seen this before in dynohub wiring?

Thanks!

Melting usually means a short, the 2 main things to check for.

The + and - wires are all hooked correctly on hub, headlight, and tail light. The Sanyo hub grounds at both the connector and frame. A little pitchfork symbol is often used for - which is the ground.

Check for a stray strand of wire between the connections on the tail light.

I wouldn't think you could melt anything with a dynohub.

Can you post a picture of the affected wire?

I'll take pictures of all connections and the melt point and will upload, checking for stray strands in the meantime.

I believe––though I'll double check––that positive and negative terminals are wired correctly. How should I check for grounding to the frame?

The axle of the hub is grounded, and (hopefully!) in contact with the fork. Some older systems took advantage of this and then used a single hot conductor for the rest of the circuit, with the ground being supplied by the fork, headset, frame, fender/stay connections. If one has any part reversed, and the ground connection to the frame and whatnot is good, you'll have problems.

I suspect it's more likely that you have some mechanical abrasion of the insulation in the regions where the wire is in contact with the frame. When you turn the handlebars, the wire needs to move. Make sure you have enough slack.

I'm also surprised that a system like this could cause anything to melt. I had thought this was a nearly perfect electrical primer, since if you stop pedaling, you'll no longer be in danger.

edit: and thanks for the note about the Sanyo. I have one, and I wondered what repercussions there would be for rolling it backwards briefly. I'm curious about it now, and I want to get in there and investigate.

Are you sure the insulation is melting, and not being abraded? It would seem much more likely for the cable to rub against the head tube than to melt, given the low current and voltage involved.

i thought it might be abrading as well. Then you might get enough localized heating to get melting. Hard to see the mechanism otherwise. Maybe a piece of heat shrink at the head tube
If there is going to be abrasion, the head tube is the place.

I had to go into my tail light to remove the connection to ground. You can check with a voltmeter between the negative connection to see if it goes to the attachment bolt. Or any place on the frame if that's more convenient

You may have developed a short due to the abrasion, which could certainly cause issues. Coaxial cable is more robust, and can deal with abrasion better/longer than 2 strand wire. I think you can buy Schmidt coax from Peter White; I pulled apart a 1/8" headphone extension, works fine, your mileage may vary. I think they specified 24 gauge as a taillight wire?

Even if the hub grounds via axle, if you have two spades connected, it's irrelevant once you have everything connected properly. There will be a "hot" wire and a ground wire (typically denoted with a stripe). Connect them to their appropriate spots on the hub. and do likewise with the taillight to the headlight.

If your light is equipped with two spades to the hub/bottle, it shouldn't ground via the frame, so you should be fine. The Toplight is also isolated from the mounts, so you shouldn't be getting any issues from the hub.

I think most taillights ground through the frame, even if they have two connections. In fact, I think my Spanninga only grounds through the frame, even though there are two wire connections

After the Luxos came out with taillight failure detection, BuM realized that they didn't sell any taillights that didn't ground to the frame. Doh!

Thanks, all. I'd already thrown out the first wire, so I unfortunately don't have any pictures of how much melting there was (about 2-3 inches!), but I suspect that there may have been abrasion which led to further melting. The border between the wire and the insulation was very smooth, so it was definitely melting.

Attached are pictures of the current wire, which is also starting to show through the insulation a bit. Hard to see, but the border is rougher, so perhaps there was abrasion that's leading to a short on the frame. However, this wire had much more slack than the first one, which leads me to believe that perhaps because I wasn't using the spades I was getting a short elsewhere. I found that the spades were quite delicate, and after they broke on a tour I simply stripped the end of the wire and pushed it through the holes on the connectors. When I get new wires I'll be sure to utilize the spades and see if the short is still happening.

IIRC they put out about 3 watts at 6 volts. That's half an amp. On thin wires, that's plenty to get pretty hot.